With the rapid growth of peacetime aviation, especially in the jet transport category, the risk of mid-air collisions and near misses has increased over the years. Although there have been improvements in control techniques and technology, all too frequently human error and lapses in control efficiency increase the probability of misses and collisions.
From 1955 the development of airborne collision avoidance systems has been the subject of studies and research. In 1987 the United States, through the Federal Aviation Administration (FAA), decided to introduce such a system on a large scale and the carriage of Traffic Alert & Collision Avoidance System (TCAS II) equipment will become mandatory at the first of Jan. 1991 for 20% of all aircraft operating in the U.S. airspace and capable of carrying 30 passengers or more, at the first of Jan. 1993 for 50%, Jan. 1, 1994 for 100%, including Part 121 air carriers.
TCAS II is a system that provides collision avoidance advisories, only in the vertical plane, by means of vertical speed commands. General developments in TCAS are given for example by Aircraft Engineering of Jan. 1988, pages 27-29. Guidelines for the implementation of TCAS II in aircraft are presented in "Airworthiness and operational approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and mode S transponders", Advisory Circular AC 20-131, Mar. 10, 1988, of the FAA. Further information can be found in "ATA Standard for TCAS II, displays, symbology and controls", ATA-STD-TCAS II/1A of Jun. 15, 1989 as well as in another document, "Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System", RTCA/DO-185.
The hardware required to implement TCAS equipment on board of an aircraft consists in a practical embodiment of a mode-S transponder, a TCAS computer, related antennes, a control panel and display devices. More specifically the equipment may comprise two display devices: a Traffic Display to alert the crew to the presence of another aircraft in the neighborhood and a Resolution Advisory Display that advises the crew of the rate of altitude change necessary to avoid a collision.
The TCAS equipment transmits interrogation signals to transponder equipped aircraft within its immediate vicinity and receives reply signals from said aircraft. By measuring the time intervals between the interrogation and the reply the TCAS computer can determine the relative range, bearing and altitude. With this information the TCAS Computer can determine a potential threat and will, if necessary, generate advisories.
Three threat levels are distinguished by TCAS. The lowest level is that of the other proximity aircraft (PA), which simply tells the flight crew that another aircraft is in the neighborhood. This advisory is displayed on the Traffic Display (TD) which is embodied as a plan position indicator (PPI).
When an other aircraft is close but not yet a hazard the second level, a traffic advisory (TA), is generated. On the Traffic Display (TD) the shape and color of the symbol representing that other aircraft will change and the audio advisory "TRAFFIC" is heard.
The third level, a resolution advisory (RA), indicates a potential collision course. Once more the shape and color of the symbol representing the intruder will change on the Traffic Display (TD). Simultaneously, on the Resolution Advisory Display (RA) the flight crew gets instructions on how to resolve the conflict. For TCAS II these instructions consist of vertical speed command only. When the RA appears a voice is heard indicating the type of RA.
It is well known, for instance from the above mentioned publications, to combine the Resolution Advisory Display with the Instantaneous Vertical Speed Indicator (IVSI) which provides data about the vertical speed of the aircraft. Such an instrument can be embodied as a conventional round dial instantaneous vertical speed indicator. Flight International of Sep. 22, 1979, pages 984 to 985, gives illustrations of such an instrument. The VSI-needle shows the rate of climb or descent of the aircraft by the position of a pointer on a circular scale. TCAS resolution advisories pointing out that the aircraft has to descend or climb at a limited rate, are displayed on the IVSI by illuminating with a red arc along the perimeter the vertical speeds that must be avoided in order to maintain safe vertical separation, in combination with a green "fly to" symbol. The pilot has to maneuver the aircraft promptly to achieve a vertical speed just out of the illuminated red arc, following the green "fly to" arc, or to maintain a vertical speed out of the illuminated red arc area, if currently not in it.
An example of a round dial display is described in the international patent application WO 89/04003 of Allied-Signal Inc., titled "Traffic Advisory Vertical Speed and Radar Display". In fact this publication describes a combination of a Traffic Display (TD) with an Instantaneous Vertical Speed Indicator (IVSI). Whenever the TCAS computer determines that a particular aircraft constitutes a collision threat the computer issues a resolution advisory which is, as indicated above, displayed on the one hand by identifying the threatening aircraft with a characterizing symbol on the PPI section of the display and on the other hand by presenting either an arrow symbol directed up or down depending on wether the resolution advisory is CLIMB or DESCEND or an arrow symbol in combination with a DON'T symbol indicating a DON'T CLIMB or a DON'T DESCEND advisory.
Instead of a separate Instantaneous Vertical Speed Indicator the vertical speed can be displayed on the Primary Flight Display (PFD) format, which is always situated on the pilot's front panel. PFD's are described for example by the publications Boeing Airliner Apr./Jun. 1987, pages 8 to 12, Aviation Week & Space Technology, Feb. 23, 1987, pages 43 to 46 and Paper 89-3361 of the AIAA Atmospheric Flight Mechanics Conference of Aug. 14-16, 1989 in Boston. A PFD indicates with analog strips, digital references and symbols at least the heading of the aircraft, the airspeed, the attitude, the altitude and the vertical speed. A vertically oriented scale or tape at the right hand of the display field, having graduations representing speed values and a movable pointer, is used to indicate the speed. In the case of Resolution Advisories a red zone shall be shown on the vertical speed scale for the forbidden vertical speeds. Thereby a green "fly to" region shall be attached to the red zone for corrective RA's.
A practical embodiment of such a primary flight display providing a variety of different data to the crew is described in the European patent application EP-A-0 324 195. This display comprises a central attitude director field for displaying a line symbol representing the artificial horizon, a pitch scale extending perpendicular to said line symbol and indicating pitch angles with reference to said line symbol, and an aircraft symbol indicating by its position along said pitch scale the momentaneous aircraft pitch attitude.
There is more information that can be provided by the PFD to the flight crew. The U.S. Pat. No. 4,910,513 of The Boeing Company relates to a PFD which visualizes next to the pitch attitude of the aircraft, the margin between the current angle of attack and the angle of attack at which onset of stall warning occurs. In other words, to escape from a critical windshear situation which is always performed by a pitch-up maneuver, the PFD is providing the pilot a visual feedback for a safe pitch-up maneuver.
An object of the invention is now to provide an improved display of the type mentioned above.